Changes

Most of the production from Wattenberg comes from Cretaceous reservoirs (Dakota, Muddy (J) Sandstone, Codell Sandstone, Niobrara Formation, Hygiene Sandstone, and Terry Sandstone). [[:file:M125-WattenbergField-Figure7.jpg|Figure 7]] is a north-south stratigraphic cross section through the central Wattenberg area. Location of the section is shown in [[:file:M125-WattenbergField-Figure8.jpg|Figure 8]]. The Wattenberg High paleostructure trends west to east across GWA and curves to the northeast in the eastern part of the mapped area. This feature was discussed by Weimer and Sonnenberg<ref name=WeimSon1982 />. This Wattenberg High is well illustrated on [[:file:M125-WattenbergField-Figure7.jpg|Figures 7]] and [[:file:M125-WattenbergField-Figure8.jpg|Figures 8]]. The thin area shown in [[:file:M125-WattenbergField-Figure8.jpg|Figure 8]] is interpreted to be because of erosional truncation and onlap of units in the upper Niobrara.

Most of the production from Wattenberg comes from Cretaceous reservoirs (Dakota, Muddy (J) Sandstone, Codell Sandstone, Niobrara Formation, Hygiene Sandstone, and Terry Sandstone). [[:file:M125-WattenbergField-Figure7.jpg|Figure 7]] is a north-south stratigraphic cross section through the central Wattenberg area. Location of the section is shown in [[:file:M125-WattenbergField-Figure8.jpg|Figure 8]]. The Wattenberg High paleostructure trends west to east across GWA and curves to the northeast in the eastern part of the mapped area. This feature was discussed by Weimer and Sonnenberg<ref name=WeimSon1982 />. This Wattenberg High is well illustrated on [[:file:M125-WattenbergField-Figure7.jpg|Figures 7]] and [[:file:M125-WattenbergField-Figure8.jpg|Figures 8]]. The thin area shown in [[:file:M125-WattenbergField-Figure8.jpg|Figure 8]] is interpreted to be because of erosional truncation and onlap of units in the upper Niobrara.

[[file:M125-WattenbergField-Figure7.jpg|center|framed|300 px|{{Figure number|7}}South to north stratigraphic cross section through Wattenberg Field. Section is datumed on top of the Niobrara Formation. Dramatic thinning of the Niobrara occurs as a result of erosional truncation and onlap onto a paleostructural feature known as the Wattenberg High. Location of the cross section is shown in [[:file:M125-WattenbergField-Figure8.jpg|Figure 8]].]]

 

file:M125-WattenbergField-Figure7.jpg|{{Figure number|7}}South to north stratigraphic cross section through Wattenberg Field. Section is datumed on top of the Niobrara Formation. Dramatic thinning of the Niobrara occurs as a result of erosional truncation and onlap onto a paleostructural feature known as the Wattenberg High. Location of the cross section is shown in [[:file:M125-WattenbergField-Figure8.jpg|Figure 8]]

[[file:M125-WattenbergField-Figure8.jpg|center|framed|300 px|{{Figure number|8}}Isopach map of J Sandstone to top of Niobrara interval. The Wattenberg High paleostructure trends west to east across Wattenberg and curves to the northeast in the eastern portion of the mapped area. CMB is an abbreviation for the Colorado Mineral Belt. GWA = Greater Wattenberg Area. Contour interval 50 ft.]]

file:M125-WattenbergField-Figure8.jpg|{{Figure number|8}}Isopach map of J Sandstone to top of Niobrara interval. The Wattenberg High paleostructure trends west to east across Wattenberg and curves to the northeast in the eastern portion of the mapped area. CMB is an abbreviation for the Colorado Mineral Belt. GWA = Greater Wattenberg Area. Contour interval 50 ft.

The reservoirs discussed in this chapter are Plainview/Lytle (Dakota) sandstones, Muddy (J) Sandstone, Codell Sandstone, Niobrara Formation, and Terry and Hygiene sandstones. The Terry and Hygiene sandstones (shallower Upper Cretaceous sandstones) production is generally assigned to the Spindle, Hambert, Aristocrat, LaPoudre, and Wattenberg fields and will be discussed later in this chapter.

The reservoirs discussed in this chapter are Plainview/Lytle (Dakota) sandstones, Muddy (J) Sandstone, Codell Sandstone, Niobrara Formation, and Terry and Hygiene sandstones. The Terry and Hygiene sandstones (shallower Upper Cretaceous sandstones) production is generally assigned to the Spindle, Hambert, Aristocrat, LaPoudre, and Wattenberg fields and will be discussed later in this chapter.

The Muddy (J) Sandstone ranges in thickness from 40 to 150 ft across Wattenberg Field and consists of two members: an older Fort Collins Member and a younger Horsetooth Member<ref name=WeimSon1982 /><ref name=WeimSon1989 /><ref> MacKenzie, D. B., 1965, [https://archives.datapages.com/data/bulletns/1965-67/data/pg/0049/0002/0150/0186.htm  Depositional environments of Muddy Sandstone, western Denver basin, Colorado]: AAPG Bulletin, v. 49, p. 186–206.</ref><ref> Weimer, R. J., S. A. Sonnenberg, G. B. C. Young, 1986, [https://archives.datapages.com/data/specpubs/resmi1/data/a066/a066/0001/0100/0143.htm Wattenberg Field, Denver Basin, Colorado], ‘’in’’ C. W. Spencer, and R. F. Mast, eds., Geology of tight gas reservoirs: [https://archives.datapages.com/data/alt-browse/aapg-special-volumes/sg24.htm AAPG Studies in Geology 24], p. 143–164.</ref> ([[:file:M125-WattenbergField-Figure7.jpg|Figures 7]], [[:file:M125-WattenbergField-Figure10.jpg|10]], [[:file:M125-WattenbergField-Figure11.jpg|11]], [[:file:M125-WattenbergField-Figure12.jpg|12]]). The Fort Collins consists of very fine- to fine-grained sandstone containing marine trace fossils and is interpreted to be a delta front deposit. The Horsetooth consists of fine- to medium-grained, well-sorted, cross-stratified sandstone. This sandstone is interpreted to have been deposited as part of a valley-fill deposit of fresh to brackish water origin. The sandstones contain carbonized wood fragments and are intercalated with siltstone and mudstone. The valley-fill model is shown in [[:file:M125-WattenbergField-Figure13.jpg|Figure 13]]. The sequence stratigraphy of the Muddy (J) Sandstone reservoir in Wattenberg field was discussed in detail by Weimer and Sonnenberg<ref name=WeimSon1989 />. Regional distribution of the Muddy (J) and valley-fill systems have been published by Weimer and Sonnenberg<ref name=WeimSon1989 />, Dolson et al.<ref> Dolson, J. C., D. S. Muller, M. J. Evetts, and J. Stein, 1991, [https://archives.datapages.com/data/bulletns/1990-91/data/pg/0075/0003/0000/0409.htm Regional paleotopographic trends and production, Muddy Sandstone (Lower Cretaceous), Central and Northern Rocky Mountains]: AAPG Bulletin, v. 75, no. 3, p. 409–435.</ref>, and others.

The Muddy (J) Sandstone ranges in thickness from 40 to 150 ft across Wattenberg Field and consists of two members: an older Fort Collins Member and a younger Horsetooth Member<ref name=WeimSon1982 /><ref name=WeimSon1989 /><ref> MacKenzie, D. B., 1965, [https://archives.datapages.com/data/bulletns/1965-67/data/pg/0049/0002/0150/0186.htm  Depositional environments of Muddy Sandstone, western Denver basin, Colorado]: AAPG Bulletin, v. 49, p. 186–206.</ref><ref> Weimer, R. J., S. A. Sonnenberg, G. B. C. Young, 1986, [https://archives.datapages.com/data/specpubs/resmi1/data/a066/a066/0001/0100/0143.htm Wattenberg Field, Denver Basin, Colorado], ‘’in’’ C. W. Spencer, and R. F. Mast, eds., Geology of tight gas reservoirs: [https://archives.datapages.com/data/alt-browse/aapg-special-volumes/sg24.htm AAPG Studies in Geology 24], p. 143–164.</ref> ([[:file:M125-WattenbergField-Figure7.jpg|Figures 7]], [[:file:M125-WattenbergField-Figure10.jpg|10]], [[:file:M125-WattenbergField-Figure11.jpg|11]], [[:file:M125-WattenbergField-Figure12.jpg|12]]). The Fort Collins consists of very fine- to fine-grained sandstone containing marine trace fossils and is interpreted to be a delta front deposit. The Horsetooth consists of fine- to medium-grained, well-sorted, cross-stratified sandstone. This sandstone is interpreted to have been deposited as part of a valley-fill deposit of fresh to brackish water origin. The sandstones contain carbonized wood fragments and are intercalated with siltstone and mudstone. The valley-fill model is shown in [[:file:M125-WattenbergField-Figure13.jpg|Figure 13]]. The sequence stratigraphy of the Muddy (J) Sandstone reservoir in Wattenberg field was discussed in detail by Weimer and Sonnenberg<ref name=WeimSon1989 />. Regional distribution of the Muddy (J) and valley-fill systems have been published by Weimer and Sonnenberg<ref name=WeimSon1989 />, Dolson et al.<ref> Dolson, J. C., D. S. Muller, M. J. Evetts, and J. Stein, 1991, [https://archives.datapages.com/data/bulletns/1990-91/data/pg/0075/0003/0000/0409.htm Regional paleotopographic trends and production, Muddy Sandstone (Lower Cretaceous), Central and Northern Rocky Mountains]: AAPG Bulletin, v. 75, no. 3, p. 409–435.</ref>, and others.

[[file:M125-WattenbergField-Figure10.jpg|center|framed|300 px|{{Figure number|10}}Type log for Wattenberg Field. Curves shown are gamma ray, spontaneous potential, resistivity, and neutron/density. Black bar in depth track indicates cored interval. Fort Collins Member designated F.C.M., and Horsetooth Member designated H.M. Note perforated interval 8160–8173 ft and associated neutron-density crossover gas effect.]]

 

file:M125-WattenbergField-Figure10.jpg|{{Figure number|10}}Type log for Wattenberg Field. Curves shown are gamma ray, spontaneous potential, resistivity, and neutron/density. Black bar in depth track indicates cored interval. Fort Collins Member designated F.C.M., and Horsetooth Member designated H.M. Note perforated interval 8160–8173 ft and associated neutron-density crossover gas effect.

[[file:M125-WattenbergField-Figure11.jpg|center|framed|300 px|{{Figure number|11}}Cored photographs for the Muddy (J) Sandstone from the Amoco #1 Rocky Mountain Fuel Well (Sec. 8, T1N-R67W). Log of cored interval shown in [[:file:M125-WattenbergField-Figure9.jpg|Figure 9]]. Muddy (J) interval is subdivided into Fort Collins and Horsetooth members. LDF = lower delta front; UDF = upper delta front; LSE = lowstand surface of erosion; TSE = transgressive surface of erosion. The well was perforated in the upper delta front, the main Muddy (J) pay in Wattenberg Field.]]

file:M125-WattenbergField-Figure11.jpg|{{Figure number|11}}Cored photographs for the Muddy (J) Sandstone from the Amoco #1 Rocky Mountain Fuel Well (Sec. 8, T1N-R67W). Log of cored interval shown in [[:file:M125-WattenbergField-Figure9.jpg|Figure 9]]. Muddy (J) interval is subdivided into Fort Collins and Horsetooth members. LDF = lower delta front; UDF = upper delta front; LSE = lowstand surface of erosion; TSE = transgressive surface of erosion. The well was perforated in the upper delta front, the main Muddy (J) pay in Wattenberg Field.

 

file:M125-WattenbergField-Figure12.jpg|{{Figure number|12}}Isopach map of Muddy (J) Sandstone interval. The Muddy (J) Sandstone ranges in thickness from 160 ft to less than 60 ft across the mapped area. The thickest area is to the northeast, and thinnest area is to the west. See [[:file:M125-WattenbergField-Figure7.jpg|Figure 7]] for cross-section A-A’. Contour interval 20 ft.

[[file:M125-WattenbergField-Figure12.jpg|center|framed|300 px|{{Figure number|12}}Isopach map of Muddy (J) Sandstone interval. The Muddy (J) Sandstone ranges in thickness from 160 ft to less than 60 ft across the mapped area. The thickest area is to the northeast, and thinnest area is to the west. See [[:file:M125-WattenbergField-Figure7.jpg|Figure 7]] for cross-section A-A’. Contour interval 20 ft.]]

file:M125-WattenbergField-Figure13.jpg|{{Figure number|Figure 13}}Valley-fill model for the Muddy (J) Sandstone (from Weimer and Sonnenberg<ref name=WeimSon1982 />). (A) Deposition of Fort Collins Member, wave-dominated deltaic progradation overlying Skull Creek Shale, (B) truncation of older marine and deltaic deposits with sea level lowering, and (C) sea level rise and back filling of valley networks with fluvial and marine strata (Horsetooth Member).

 

[[file:M125-WattenbergField-Figure13.jpg|center|framed|300 px|{{Figure number|Figure 13}}Valley-fill model for the Muddy (J) Sandstone (from Weimer and Sonnenberg<ref name=WeimSon1982 />). (A) Deposition of Fort Collins Member, wave-dominated deltaic progradation overlying Skull Creek Shale, (B) truncation of older marine and deltaic deposits with sea level lowering, and (C) sea level rise and back filling of valley networks with fluvial and marine strata (Horsetooth Member).]]

The Muddy (J) Sandstone ranges in thickness from 60 to 160 ft across the mapped area; the reservoir is productive over an area of 800,000 acres at depths of 7400–8500 ft ([[:file:M125-WattenbergField-Figure12.jpg|Figure 12]]). Reservoir parameters are as follows: pay thickness 10–50 ft, porosity range 8%–12%, permeability range 0.1–0.05 md, reservoir temperature 260°F, reservoir pressure 2750 psig, average water saturation 44%, and initial potentials 100–3575 MCFGPD. Initial reserves were estimated by Matuszczak<ref name= Matuszczak /> to be 1.1 TCFG from 283,000 acres. Updated reserves are 3.1 TCFG from 800,000 acres. Original well spacing was 320 acres, and current spacing is approximately 20 acres.

The Muddy (J) Sandstone ranges in thickness from 60 to 160 ft across the mapped area; the reservoir is productive over an area of 800,000 acres at depths of 7400–8500 ft ([[:file:M125-WattenbergField-Figure12.jpg|Figure 12]]). Reservoir parameters are as follows: pay thickness 10–50 ft, porosity range 8%–12%, permeability range 0.1–0.05 md, reservoir temperature 260°F, reservoir pressure 2750 psig, average water saturation 44%, and initial potentials 100–3575 MCFGPD. Initial reserves were estimated by Matuszczak<ref name= Matuszczak /> to be 1.1 TCFG from 283,000 acres. Updated reserves are 3.1 TCFG from 800,000 acres. Original well spacing was 320 acres, and current spacing is approximately 20 acres.

Most of the production comes from the Fort Collins Member (fine-grained delta front sandstone), which ranges in thickness from less than 25 ft to more than 100 ft ([[:file:M125-WattenbergField-Figure11.jpg|Figures 11]], ([[:file:M125-WattenbergField-Figure12.jpg|12]], [[:file:M125-WattenbergField-Figure13.jpg|13]], [[:file:M125-WattenbergField-Figure14.jpg|14]], [[:file:M125-WattenbergField-Figure15.jpg|15]]). Thin areas in the Fort Collins are caused mainly from erosional incisement by Horsetooth valleys. The main pay section in the Fort Collins varies from 10 to 20 ft in thickness. The Fort Collins sandstones consist of approximately 80% quartz, 10% argillaceous matrix, 5% rock fragments, and 5% feldspar. Clay is the main matrix material, which is both detrital and diagenetic. The sandstones are bioturbated, and the amount of bioturbation decreases vertically in abundance. Higher energy conditions in the upper Fort Collins winnow out the clay fraction. Trace fossils change vertically from largely deposit feeders (lower Fort Collins) to suspension-feeding organisms (upper Fort Collins). Porosity is mainly intergranular with minor microporosity found in the matrix. The most important diagenetic event in the Fort Collins is formation of quartz overgrowths. The percentage of quartz overgrowths decreases as the clay content increases. Pores are triangular in shape because of the quartz overgrowths. [[:file:M125-WattenbergField-Figure15.jpg|Figure 15]] illustrates location of wells that have cumulative production in excess of 500 MMCFG. The production sweet spot coincides with an isopach thick in the Fort Collins Member. Also important to production is the Wattenberg temperature anomaly, which is represented by the vitrinite reflectance contours.

Most of the production comes from the Fort Collins Member (fine-grained delta front sandstone), which ranges in thickness from less than 25 ft to more than 100 ft ([[:file:M125-WattenbergField-Figure11.jpg|Figures 11]], ([[:file:M125-WattenbergField-Figure12.jpg|12]], [[:file:M125-WattenbergField-Figure13.jpg|13]], [[:file:M125-WattenbergField-Figure14.jpg|14]], [[:file:M125-WattenbergField-Figure15.jpg|15]]). Thin areas in the Fort Collins are caused mainly from erosional incisement by Horsetooth valleys. The main pay section in the Fort Collins varies from 10 to 20 ft in thickness. The Fort Collins sandstones consist of approximately 80% quartz, 10% argillaceous matrix, 5% rock fragments, and 5% feldspar. Clay is the main matrix material, which is both detrital and diagenetic. The sandstones are bioturbated, and the amount of bioturbation decreases vertically in abundance. Higher energy conditions in the upper Fort Collins winnow out the clay fraction. Trace fossils change vertically from largely deposit feeders (lower Fort Collins) to suspension-feeding organisms (upper Fort Collins). Porosity is mainly intergranular with minor microporosity found in the matrix. The most important diagenetic event in the Fort Collins is formation of quartz overgrowths. The percentage of quartz overgrowths decreases as the clay content increases. Pores are triangular in shape because of the quartz overgrowths. [[:file:M125-WattenbergField-Figure15.jpg|Figure 15]] illustrates location of wells that have cumulative production in excess of 500 MMCFG. The production sweet spot coincides with an isopach thick in the Fort Collins Member. Also important to production is the Wattenberg temperature anomaly, which is represented by the vitrinite reflectance contours.

[[file:M125-WattenbergField-Figure14.jpg|center|framed|300 px|{{Figure number|14}}Isopach map of the Fort Collins Member of the Muddy (J) Sandstone. The Fort Collins ranges in thickness from 25 to 100 ft across the mapped area. The thickest area is located south of Greeley. See [[:file:M125-WattenbergField-Figure7.jpg|Figure 7]] for cross-section A-A’. The best production occurs between Broomfield and Greeley where the total Fort Collins interval is 75–100 ft thick. GWA = Greater Wattenberg Area. Contour interval 25 ft.]]

 

file:M125-WattenbergField-Figure14.jpg|{{Figure number|14}}Isopach map of the Fort Collins Member of the Muddy (J) Sandstone. The Fort Collins ranges in thickness from 25 to 100 ft across the mapped area. The thickest area is located south of Greeley. See [[:file:M125-WattenbergField-Figure7.jpg|Figure 7]] for cross-section A-A’. The best production occurs between Broomfield and Greeley where the total Fort Collins interval is 75–100 ft thick. GWA = Greater Wattenberg Area. Contour interval 25 ft.]]

[[file:M125-WattenbergField-Figure15.jpg|center|framed|300 px|{{Figure number|15}}Isopach map of the Fort Collins Member of the Muddy (J) Sandstone. Contour interval is 25 ft. Also shown are Muddy (J) wells, which have cumulative production greater than 500 MMCFG. The thickness in Fort Collins is the sweet spot area for production. Also shown are Ro values for shales above the Muddy (J). Vitrinite reflectance contours modified from Higley and Cox<ref name=HigCox>Higley, D. K., and D. O. Cox, 2007, Oil and gas exploration and development along the front range in the Denver Basin of Colorado, Nebraska, and Wyoming, ‘’in’’ D. K. Higley, compiler, Petroleum systems and assessment of undiscovered oil and gas in the Denver Basin Province, Colorado, Kansas, Nebraska, South Dakota, and Wyoming—USGS Province 39: USGS Digital Data Series DDS–69–P, 41 p.</ref> and Smagala et al.<ref name=Smag>Smagala, T. M., C. A. Brown, and G. L. Nydegger, 1984, Log-derived indicator of thermal maturity, Niobrara Formation, Denver Basin, Colorado, Nebraska, Wyoming, in J. Woodward, F. F. Meissner, and J. C. Clayton, eds., Hydrocarbon source rocks of the greater Rocky Mountain region: RMAG, Denver, Colorado, p. 355–363.</ref>. GWA = Greater Wattenberg Area. The best Muddy (J) production generally coincides with the highest maturity areas and thickest Fort Collins.]]

file:M125-WattenbergField-Figure15.jpg|{{Figure number|15}}Isopach map of the Fort Collins Member of the Muddy (J) Sandstone. Contour interval is 25 ft. Also shown are Muddy (J) wells, which have cumulative production greater than 500 MMCFG. The thickness in Fort Collins is the sweet spot area for production. Also shown are Ro values for shales above the Muddy (J). Vitrinite reflectance contours modified from Higley and Cox<ref name=HigCox>Higley, D. K., and D. O. Cox, 2007, Oil and gas exploration and development along the front range in the Denver Basin of Colorado, Nebraska, and Wyoming, ‘’in’’ D. K. Higley, compiler, Petroleum systems and assessment of undiscovered oil and gas in the Denver Basin Province, Colorado, Kansas, Nebraska, South Dakota, and Wyoming—USGS Province 39: USGS Digital Data Series DDS–69–P, 41 p.</ref> and Smagala et al.<ref name=Smag>Smagala, T. M., C. A. Brown, and G. L. Nydegger, 1984, Log-derived indicator of thermal maturity, Niobrara Formation, Denver Basin, Colorado, Nebraska, Wyoming, in J. Woodward, F. F. Meissner, and J. C. Clayton, eds., Hydrocarbon source rocks of the greater Rocky Mountain region: RMAG, Denver, Colorado, p. 355–363.</ref>. GWA = Greater Wattenberg Area. The best Muddy (J) production generally coincides with the highest maturity areas and thickest Fort Collins.

The Horsetooth Member ranges in thickness from less than 25 ft to more than 125 ft across Wattenberg Field ([[:file:M125-WattenbergField-Figure16.jpg|Figure 16]]). The sandstones are fine to medium grained, well sorted, and rounded and consist of 57%–80% quartz, 5%–10% rock fragments, and 5% feldspar. Porosity is intergranular with minor microporosity and intraparticle porosity. The Horsetooth Member is thickest in the northeast, southeast, and southwest parts of the mapped area. Diagenesis is similar to the Fort Collins Member with abundant silica cement along with smectite, chlorite, and kaolinite clay pore fillings. The clays can contribute to low-resistivity pay in the Horsetooth Member.

The Horsetooth Member ranges in thickness from less than 25 ft to more than 125 ft across Wattenberg Field ([[:file:M125-WattenbergField-Figure16.jpg|Figure 16]]). The sandstones are fine to medium grained, well sorted, and rounded and consist of 57%–80% quartz, 5%–10% rock fragments, and 5% feldspar. Porosity is intergranular with minor microporosity and intraparticle porosity. The Horsetooth Member is thickest in the northeast, southeast, and southwest parts of the mapped area. Diagenesis is similar to the Fort Collins Member with abundant silica cement along with smectite, chlorite, and kaolinite clay pore fillings. The clays can contribute to low-resistivity pay in the Horsetooth Member.

The Codell in Wattenberg ranges in thickness from a wedge-edge to more than 30 ft (([[:file:M125-WattenbergField-Figure18.jpg|Figure 18]]). The Codell thickens westward across the field. An isopach map of Codell Sandstone greater than 6% porosity is shown in [[:file:M125-WattenbergField-Figure19.jpg|Figure 19]]. Porosity thickness declines to the east and thickens to the west.

The Codell in Wattenberg ranges in thickness from a wedge-edge to more than 30 ft (([[:file:M125-WattenbergField-Figure18.jpg|Figure 18]]). The Codell thickens westward across the field. An isopach map of Codell Sandstone greater than 6% porosity is shown in [[:file:M125-WattenbergField-Figure19.jpg|Figure 19]]. Porosity thickness declines to the east and thickens to the west.

[[file:M125-WattenbergField-Figure18.jpg|center|framed|300 px|{{Figure number|18}}Isopach map of the Codell Sandstone; contour interval 10 ft. Codell ranges in thickness from 5 to 30 ft across the mapped area. Thickest area is to the west. Codell is a low-resistivity, low-contrast pay as shown on [[:file:M125-WattenbergField-Figure20.jpg|Figure 20]]. GWA = Greater Wattenberg Area. Blue dashed lines show area where Codell is productive. Red dashed lines show Codell overpressured area (>0.5 psi/ft). Contour interval 5 ft.]]

 

file:M125-WattenbergField-Figure18.jpg|{{Figure number|18}}Isopach map of the Codell Sandstone; contour interval 10 ft. Codell ranges in thickness from 5 to 30 ft across the mapped area. Thickest area is to the west. Codell is a low-resistivity, low-contrast pay as shown on [[:file:M125-WattenbergField-Figure20.jpg|Figure 20]]. GWA = Greater Wattenberg Area. Blue dashed lines show area where Codell is productive. Red dashed lines show Codell overpressured area (>0.5 psi/ft). Contour interval 5 ft.

[[file:M125-WattenbergField-Figure19.jpg|center|framed|300 px|{{Figure number|19}}Isopach map of the Codell Sandstone, porosity greater than 6% using 2.68 matrix density; contour interval 5 ft. Porous Codell ranges in thickness from 5 to 25 ft across the mapped area. Thickest area is to the west. Codell pressure greater than 0.5 psi within orange dashed area. Synclinal axis at Codell level shown by dashed gray line. Outline of where Codell produces shown by dashed blue line. GWA = Greater Wattenberg Area.]]

file:M125-WattenbergField-Figure19.jpg|{{Figure number|19}}Isopach map of the Codell Sandstone, porosity greater than 6% using 2.68 matrix density; contour interval 5 ft. Porous Codell ranges in thickness from 5 to 25 ft across the mapped area. Thickest area is to the west. Codell pressure greater than 0.5 psi within orange dashed area. Synclinal axis at Codell level shown by dashed gray line. Outline of where Codell produces shown by dashed blue line. GWA = Greater Wattenberg Area.

The Codell Sandstone in Wattenberg is an impermeable, bioturbated, fine-grained marine shelf sandstone<ref name=WeimSonnen1983 />. Occasional thin hummocky cross-stratified beds (1–2 ft thick) are present in cores and the outcrop sections of the Codell ([[:file:M125-WattenbergField-Figure20.jpg|Figures 20]], [[:file:M125-WattenbergField-Figure21.jpg|21]]). The Codell Sandstone in Wattenberg is poorly to moderately sorted, very fine- to fine-grained sublitharenites and subarkose. Slight to moderate compaction and ductile grain deformation is common. Partial dissolution of chert and mica has created minor amounts of secondary porosity. Cements include mixed layer illite/smectite, illite, chlorite, quartz overgrowths, and calcite. Detrital clay further limits reservoir quality. The Codell in Wattenberg is a classic low-resistivity, low-contrast pay (LRLC; [[:file:M125-WattenbergField-Figure7.jpg|Figures 7]], [[:file:M125-WattenbergField-Figure20.jpg|20]]).

The Codell Sandstone in Wattenberg is an impermeable, bioturbated, fine-grained marine shelf sandstone<ref name=WeimSonnen1983 />. Occasional thin hummocky cross-stratified beds (1–2 ft thick) are present in cores and the outcrop sections of the Codell ([[:file:M125-WattenbergField-Figure20.jpg|Figures 20]], [[:file:M125-WattenbergField-Figure21.jpg|21]]). The Codell Sandstone in Wattenberg is poorly to moderately sorted, very fine- to fine-grained sublitharenites and subarkose. Slight to moderate compaction and ductile grain deformation is common. Partial dissolution of chert and mica has created minor amounts of secondary porosity. Cements include mixed layer illite/smectite, illite, chlorite, quartz overgrowths, and calcite. Detrital clay further limits reservoir quality. The Codell in Wattenberg is a classic low-resistivity, low-contrast pay (LRLC; [[:file:M125-WattenbergField-Figure7.jpg|Figures 7]], [[:file:M125-WattenbergField-Figure20.jpg|20]]).

[[file:M125-WattenbergField-Figure20.jpg|center|framed|300 px|{{Figure number|20}}(A) Type log Codell Sandstone, Wattenberg Field. Gamma ray, spontaneous potential, resistivity, neutron, and density curves shown. Codell is a low resistivity, low contrast pay. (B) X-ray diffraction data for the Dome Frank core<ref name=USGS>[http://geology.cr.usgs.gov/crc/ USGS Core Research Center], 2015, accessed February 4, 2021.</ref>.]]

 

file:M125-WattenbergField-Figure20.jpg|{{Figure number|20}}(A) Type log Codell Sandstone, Wattenberg Field. Gamma ray, spontaneous potential, resistivity, neutron, and density curves shown. Codell is a low resistivity, low contrast pay. (B) X-ray diffraction data for the Dome Frank core<ref name=USGS>[http://geology.cr.usgs.gov/crc/ USGS Core Research Center], 2015, accessed February 4, 2021.</ref>.

[[file:M125-WattenbergField-Figure21.jpg|center|framed|300 px|{{Figure number|21}}Core photographs of Dome Frank Codell core. Two distinct facies are present in core: (1) bioturbated sandstone and (2) low-angle cross stratified (hummocky beds). Unconformities are present at the top and base of Codell. From USGS Core Research Center<ref name=USGS />.]]

file:M125-WattenbergField-Figure21.jpg|{{Figure number|21}}Core photographs of Dome Frank Codell core. Two distinct facies are present in core: (1) bioturbated sandstone and (2) low-angle cross stratified (hummocky beds). Unconformities are present at the top and base of Codell. From USGS Core Research Center<ref name=USGS />.

The Codell has very little gamma-ray response and very little spontaneous potential (SP) response. Abnormally high porosity readings occur on density logs as a result of gas saturation ([[:file:M125-WattenbergField-Figure7.jpg|Figures 7]], [[:file:M125-WattenbergField-Figure20.jpg|20]]). Current development activity in Wattenberg Field includes horizontal wells with multistage hydraulic fracturing. The low resistivity is because of clay content of the sandstones. Values of porosity and permeability are 14% and 0.05 md for Niobrara/Codell wells in Wattenberg Field<ref name=HigCox /><ref> Higley, D. K., D. O. Cox, and R. J. Weimer, 2003, [https://archives.datapages.com/data/bulletns/2003/01jan/0015/0015.htm Petroleum system and production characteristics of the Muddy (J) Sandstone (Lower Cretaceous) Wattenberg continuous gas field, Denver Basin, Colorado]: AAPG Bulletin, v. 87, no. 1, p. 15–37.</ref>. Based on core testing, the Codell Sandstone in Wattenberg Field has an average porosity and permeability of 14% and 0.1 md, respectively<refname=HigCox />.

The Codell has very little gamma-ray response and very little spontaneous potential (SP) response. Abnormally high porosity readings occur on density logs as a result of gas saturation ([[:file:M125-WattenbergField-Figure7.jpg|Figures 7]], [[:file:M125-WattenbergField-Figure20.jpg|20]]). Current development activity in Wattenberg Field includes horizontal wells with multistage hydraulic fracturing. The low resistivity is because of clay content of the sandstones. Values of porosity and permeability are 14% and 0.05 md for Niobrara/Codell wells in Wattenberg Field<ref name=HigCox /><ref> Higley, D. K., D. O. Cox, and R. J. Weimer, 2003, [https://archives.datapages.com/data/bulletns/2003/01jan/0015/0015.htm Petroleum system and production characteristics of the Muddy (J) Sandstone (Lower Cretaceous) Wattenberg continuous gas field, Denver Basin, Colorado]: AAPG Bulletin, v. 87, no. 1, p. 15–37.</ref>. Based on core testing, the Codell Sandstone in Wattenberg Field has an average porosity and permeability of 14% and 0.1 md, respectively<refname=HigCox />.

The Niobrara consists of four limestone (chalk) units and three intervening marl intervals ([[:file:M125-WattenbergField-Figure7.jpg|Figures 7]], [[:file:M125-WattenbergField-Figure22.jpg|22]], [[:file:M125-WattenbergField-Figure23.jpg|23]], [[:file:M125-WattenbergField-Figure24.jpg|24]]). The lower limestone is named the Fort Hays, and the overlying units are grouped together as the Smoky Hill Member. The chalk units are referred to in descending order as the A, B, C, and Fort Hays. Erosional unconformities exist at the top and base of the Niobrara. The upper unconformity removes the upper chalk bed in some areas of the Wattenberg Field. The B and C chalks are the main focus of horizontal drilling by operators in the field.

The Niobrara consists of four limestone (chalk) units and three intervening marl intervals ([[:file:M125-WattenbergField-Figure7.jpg|Figures 7]], [[:file:M125-WattenbergField-Figure22.jpg|22]], [[:file:M125-WattenbergField-Figure23.jpg|23]], [[:file:M125-WattenbergField-Figure24.jpg|24]]). The lower limestone is named the Fort Hays, and the overlying units are grouped together as the Smoky Hill Member. The chalk units are referred to in descending order as the A, B, C, and Fort Hays. Erosional unconformities exist at the top and base of the Niobrara. The upper unconformity removes the upper chalk bed in some areas of the Wattenberg Field. The B and C chalks are the main focus of horizontal drilling by operators in the field.

[[file:M125-WattenbergField-Figure22.jpg|center|framed|300 px|{{Figure number|22}}Isopach Niobrara Formation; contour interval 50 ft. Axis of Wattenberg High shown in red. Niobrara ranges in thickness from 250 ft to more than 400 ft in mapped area. Red dashed lines show area of Niobrara producers. GWA = Greater Wattenberg Area.]]

 

file:M125-WattenbergField-Figure22.jpg|{{Figure number|22}}Isopach Niobrara Formation; contour interval 50 ft. Axis of Wattenberg High shown in red. Niobrara ranges in thickness from 250 ft to more than 400 ft in mapped area. Red dashed lines show area of Niobrara producers. GWA = Greater Wattenberg Area.

[[file:M125-WattenbergField-Figure23.jpeg|center|framed|300 px|{{Figure number|23}}Isopach Niobrara Formation; contour interval 50 ft. Niobrara ranges in thickness from 250 ft to more than 400 ft in mapped area. Ro contours modified from Smagala et al. .<ref name=Smag /> and Higley and Cox<refname=HigCox />. Outline of where Niobrara produces shown by dashed orange line. Note coincidence of isopach thin area with Ro anomaly. GWA = Greater Wattenberg Area.]]

file:M125-WattenbergField-Figure23.jpeg|{{Figure number|23}}Isopach Niobrara Formation; contour interval 50 ft. Niobrara ranges in thickness from 250 ft to more than 400 ft in mapped area. Ro contours modified from Smagala et al. .<ref name=Smag /> and Higley and Cox<refname=HigCox />. Outline of where Niobrara produces shown by dashed orange line. Note coincidence of isopach thin area with Ro anomaly. GWA = Greater Wattenberg Area.

 

file:M125-WattenbergField-Figure24.jpg|{{Figure number|24}}Type log for Niobrara Formation. Well was cored in the B and C chalk beds. Chalks generally have low gamma ray readings, high resistivity, and porosity development. This well was completed in the Codell Sandstone. Reservoirs in the Niobrara are the A, B, C, and Fort Hays chalk intervals. Source beds for the Niobrara include the Sharon Springs, Niobrara chalks, and also A marl, B marl, and C marl. The D marl is organic lean in Wattenberg area. The chalks have been targeted with vertical wells from 1981 through 2010. The play has now become a horizontal play targeting chalks and marls from 2010 to the present.

[[file:M125-WattenbergField-Figure24.jpg|center|framed|300 px|{{Figure number|24}}Type log for Niobrara Formation. Well was cored in the B and C chalk beds. Chalks generally have low gamma ray readings, high resistivity, and porosity development. This well was completed in the Codell Sandstone. Reservoirs in the Niobrara are the A, B, C, and Fort Hays chalk intervals. Source beds for the Niobrara include the Sharon Springs, Niobrara chalks, and also A marl, B marl, and C marl. The D marl is organic lean in Wattenberg area. The chalks have been targeted with vertical wells from 1981 through 2010. The play has now become a horizontal play targeting chalks and marls from 2010 to the present.]]

The Niobrara ranges in thickness from 300 to 400 ft across the field area ([[:file:M125-WattenbergField-Figure22.jpg|Figures 22]], [[:file:M125-WattenbergField-Figure23.jpg|23]]). The thickness changes in the Niobrara were described by Weimer and Sonnenberg<ref name=WeimSon1982 />. The west to east thin area shown on [[:file:M125-WattenbergField-Figure22.jpg|Figure 22]] was thought to be caused by a paleostructural feature of similar orientation (i.e., basement fault block). The thinning is due in part to erosional truncation of the A chalk unit over the broad east–west trending area.  [[:file:M125-WattenbergField-Figure7.jpg|Figure 7]] is a north–south stratigraphic cross section datumed on top of the Niobrara. Thickness changes in individual Niobrara chalks and marls units in the past have been noted by several authors and attributed to chalk bar formation (associated with bottom water currents), compensational stacking, paleotopography effects (low areas being isopach thicks and high areas being isopach thins), and submarine current erosion and scour. Numerous normal faults present within the Niobrara Formation create small circular isopach thins on the various isopach maps (bull’s-eyes). The normal faults create tremendous reservoir heterogeneity and potential problems for drillers trying to “stay in zone” in horizontal laterals. [[:file:M125-WattenbergField-Figure23.jpg|Figure 23]] illustrates the location of the Wattenberg temperature anomaly superimposed on the Niobrara isopach map. The outline area of where the Niobrara produces is also shown on [[:file:M125-WattenbergField-Figure23.jpg|Figure 23]].

The Niobrara ranges in thickness from 300 to 400 ft across the field area ([[:file:M125-WattenbergField-Figure22.jpg|Figures 22]], [[:file:M125-WattenbergField-Figure23.jpg|23]]). The thickness changes in the Niobrara were described by Weimer and Sonnenberg<ref name=WeimSon1982 />. The west to east thin area shown on [[:file:M125-WattenbergField-Figure22.jpg|Figure 22]] was thought to be caused by a paleostructural feature of similar orientation (i.e., basement fault block). The thinning is due in part to erosional truncation of the A chalk unit over the broad east–west trending area.  [[:file:M125-WattenbergField-Figure7.jpg|Figure 7]] is a north–south stratigraphic cross section datumed on top of the Niobrara. Thickness changes in individual Niobrara chalks and marls units in the past have been noted by several authors and attributed to chalk bar formation (associated with bottom water currents), compensational stacking, paleotopography effects (low areas being isopach thicks and high areas being isopach thins), and submarine current erosion and scour. Numerous normal faults present within the Niobrara Formation create small circular isopach thins on the various isopach maps (bull’s-eyes). The normal faults create tremendous reservoir heterogeneity and potential problems for drillers trying to “stay in zone” in horizontal laterals. [[:file:M125-WattenbergField-Figure23.jpg|Figure 23]] illustrates the location of the Wattenberg temperature anomaly superimposed on the Niobrara isopach map. The outline area of where the Niobrara produces is also shown on [[:file:M125-WattenbergField-Figure23.jpg|Figure 23]].

Production occurs in the Upper Cretaceous (Campanian) Hygiene and Terry sandstones in the Wattenberg area ([[:file:M125-WattenbergField-Figure26.jpg|Figures 26]], [[:file:M125-WattenbergField-Figure27.jpg|27]], [[:file:M125-WattenbergField-Figure28.jpg|28]]). Most of the production is from the Terry Sandstone. The sandstones are interpreted to be marine sandstones enclosed in finer-grained mudstones<ref name=Porter1989 /><ref>Moredock, D. E., and S. J. Williams, 1976, Upper Cretaceous Terry and Hygiene Sandstones, Singletree, Spindle, and Surrey Fields, Weld County, Colorado, ‘’in’’ R. C. Epis, and R. J. Weimer, eds., Studies in Colorado field geology: CSM Professional Contribution 8, p. 264–274.</ref><ref name=PortWeim1982>Porter, K., and R. J. Weimer, 1982, [https://archives.datapages.com/data/bulletns/1982-83/data/pg/0066/0012/2500/2543.htm Diagenetic sequence related to structural history and petroleum accumulation: Spindle field, Colorado]: AAPG Bulletin, v. 66, no. 12, p. 2543–2560.</ref>. Sandstones are glauconitic, feldspathic litharenites. The depositional environment was interpreted by Porter<ref name=Porter1989 /> to be offshore marine bars ([[:file:M125-WattenbergField-Figure27.jpg|Figure 27]]). Coarsening upward sequences consist of three facies: bioturbated sandy mudstones; burrowed to nonburrowed, thin-bedded, rippled to cross-stratified sandstone; and fine- to medium-grained, cross-stratified sandstone. Sandstone facies are designated as sandy shelf, bar margin, and central bar. The marine bars in the Terry trend northwest-southeast. Bars are 2–4 miles wide and 8–16 miles long. The nature of the trap is stratigraphic. Source rocks are the Sharon Springs Member of the Pierre Shale and the Niobrara Formation.

Production occurs in the Upper Cretaceous (Campanian) Hygiene and Terry sandstones in the Wattenberg area ([[:file:M125-WattenbergField-Figure26.jpg|Figures 26]], [[:file:M125-WattenbergField-Figure27.jpg|27]], [[:file:M125-WattenbergField-Figure28.jpg|28]]). Most of the production is from the Terry Sandstone. The sandstones are interpreted to be marine sandstones enclosed in finer-grained mudstones<ref name=Porter1989 /><ref>Moredock, D. E., and S. J. Williams, 1976, Upper Cretaceous Terry and Hygiene Sandstones, Singletree, Spindle, and Surrey Fields, Weld County, Colorado, ‘’in’’ R. C. Epis, and R. J. Weimer, eds., Studies in Colorado field geology: CSM Professional Contribution 8, p. 264–274.</ref><ref name=PortWeim1982>Porter, K., and R. J. Weimer, 1982, [https://archives.datapages.com/data/bulletns/1982-83/data/pg/0066/0012/2500/2543.htm Diagenetic sequence related to structural history and petroleum accumulation: Spindle field, Colorado]: AAPG Bulletin, v. 66, no. 12, p. 2543–2560.</ref>. Sandstones are glauconitic, feldspathic litharenites. The depositional environment was interpreted by Porter<ref name=Porter1989 /> to be offshore marine bars ([[:file:M125-WattenbergField-Figure27.jpg|Figure 27]]). Coarsening upward sequences consist of three facies: bioturbated sandy mudstones; burrowed to nonburrowed, thin-bedded, rippled to cross-stratified sandstone; and fine- to medium-grained, cross-stratified sandstone. Sandstone facies are designated as sandy shelf, bar margin, and central bar. The marine bars in the Terry trend northwest-southeast. Bars are 2–4 miles wide and 8–16 miles long. The nature of the trap is stratigraphic. Source rocks are the Sharon Springs Member of the Pierre Shale and the Niobrara Formation.

[[file:M125-WattenbergField-Figure26.jpg|center|framed|300 px|{{Figure number|26}}Structure contour of Terry Sandstone marker bentonite bed. Contour interval 100 ft. Map shows Terry (green dots), Hygiene (red dots), and Terry/Hygiene commingled (pink dots) production. Vast majority of production comes from Terry Sandstone (green dots). Type log shown in this figure. Production curve shown in [[:file:M125-WattenbergField-Figure27.jpg|Figure 27]]. Field names from scout ticket reports. The Surrey, Spindle, and Singletree fields are now combined together into the Spindle Field. GWA = Greater Wattenberg Area. These fields are regarded as unconventional accumulations of stratigraphically trapped oil and gas. Fields are shallow pool accumulations in the Wattenberg Field.]]

 

file:M125-WattenbergField-Figure26.jpg|{{Figure number|26}}Structure contour of Terry Sandstone marker bentonite bed. Contour interval 100 ft. Map shows Terry (green dots), Hygiene (red dots), and Terry/Hygiene commingled (pink dots) production. Vast majority of production comes from Terry Sandstone (green dots). Type log shown in this figure. Production curve shown in [[:file:M125-WattenbergField-Figure27.jpg|Figure 27]]. Field names from scout ticket reports. The Surrey, Spindle, and Singletree fields are now combined together into the Spindle Field. GWA = Greater Wattenberg Area. These fields are regarded as unconventional accumulations of stratigraphically trapped oil and gas. Fields are shallow pool accumulations in the Wattenberg Field.

[[file:M125-WattenbergField-Figure27.jpg|center|framed|300 px|{{Figure number|27}}(A) Type log for Terry and Hygiene sandstones, Wattenberg Field area. (B) Paleography for Upper Campanian. (C) Offshore shallow shelf model for sand bar development. Modified from Porter and Weimer<ref name=PortWeim1982 />.]]

file:M125-WattenbergField-Figure27.jpg|{{Figure number|27}}(A) Type log for Terry and Hygiene sandstones, Wattenberg Field area. (B) Paleography for Upper Campanian. (C) Offshore shallow shelf model for sand bar development. Modified from Porter and Weimer<ref name=PortWeim1982 />.

 

file:M125-WattenbergField-Figure28.jpg|{{Figure number|28}}Production curve for Terry and Hygiene sandstones from Wattenberg—Terry, Spindle, Singletree, Surrey, Hambert, New Windsor, and LaPoudre fields.

[[file:M125-WattenbergField-Figure28.jpg|center|framed|300 px|{{Figure number|28}}Production curve for Terry and Hygiene sandstones from Wattenberg—Terry, Spindle, Singletree, Surrey, Hambert, New Windsor, and LaPoudre fields.]]

Porosity averages 14%; permeability averages 2 md<ref name=Porter1989 />. Producing depths are 4200–5200 ft. Reservoir thickness is 20–25 ft. Original field pressure is 1450 psi, drive mechanism is solution-gas drive, oil gravity is 42° API, and minimum water saturation is 30%. Average initial potential from wells is 125 BOPD. The Terry and Hygiene fields are regarded as unconventional stratigraphic traps of oil and gas. They are deemed unconventional because the reservoirs require fracture stimulation and the regional aspect of the hydrocarbon accumulations along with the general lack of produced water. The fracture stimulations are quite variable and generally range from 50,000 to 120,000 gallons fluid and 60,000 to 400,000 lb of sand.

Porosity averages 14%; permeability averages 2 md<ref name=Porter1989 />. Producing depths are 4200–5200 ft. Reservoir thickness is 20–25 ft. Original field pressure is 1450 psi, drive mechanism is solution-gas drive, oil gravity is 42° API, and minimum water saturation is 30%. Average initial potential from wells is 125 BOPD. The Terry and Hygiene fields are regarded as unconventional stratigraphic traps of oil and gas. They are deemed unconventional because the reservoirs require fracture stimulation and the regional aspect of the hydrocarbon accumulations along with the general lack of produced water. The fracture stimulations are quite variable and generally range from 50,000 to 120,000 gallons fluid and 60,000 to 400,000 lb of sand.